Electric precipitators



United States Patent O Filed Ian. 27, 1960, Ser. No. 5,007 6 Claims.(Cl. SiS-105) This invention relates to electric precipitators and moreparticularly to ones adapted to remove dust from a dust laden gasstream. Such precipitators have emitting electrodes, which arecustomarily wires, in the gas stream and electrically groundedcollecting electrodes which generally form the walls of the passagethrough which the stream is passed. The emitting wires and collectingplates are thus associated in spaced relation from each other in thepassage. These are a multiplicity of wires and plates, and aprecipitator has a multiplicity of passages so that the volume ofincoming gas can be split up among the passages for simultaneoustreatment. Direct current is applied to the emitting wires and thevoltage is sutlicient to ionize the gas forming a corona and charge thedust particles negatively. The electric eld causes the particles totravel to the collecting plates where they form a dust layer. The higherthe voltage, the more ions are formed, the greater the force, the higherthe current and the more eicient the cleaning operation. It follows thatthe higher the voltage in a precipitator, the more ecient is itsoperation.

However, there is a ilimit to the voltage usable. The breakdown voltageof the path of the current from the emitting wire to the collectingplate must not be eX- ceeded or an arc-over will occur and dustcollection will stop.

It has been noted that the path of the direct current from wire to plateshows the current must pass not only through the gas but also throughthe layer of dust on the surface of the plate. It is found that thevoltage drop through the dust layer is more rapid than through the gasand that the dust has certain other qualities hereinafter referred towhich have enabled the inventor to make an improved precipitator whichavoids arc-overs. According to the invention, the temperature of the gasis used to regulate conditions according to the dust to automaticallykeep the voltage as close to the normal or most ecient voltage aspossible from moment to moment.

In the drawings,

FIG. l is a diagrammatic sketch in vertical section of one end of theprecipitator viewed from the side, in which the invention can beembodied.

FIG. 2 is a detailed view in plan of one of the passages in theprecipitator of FIG. 1.

FIG. 3 is a diagram of the voltage drop situation at the wire and platewith a kilo volt scale beside it.

FIG. 4 is a graph in which the curve illustrates the relationship of theresistivity of a sample of ily ash and the temperature of the gas.

FIG. 5 is a schematic drawing of the temperature sensing unit, voltagecontrol equipment and the gas stream.

As shown in the embodiment of FIGS. l and 2, there are a great number ofemitting wires 1 serving as emitting electrodes. There may be as many asthousands of wires in a precipitator divided among a number of parallelpassages 2, the walls of the passages being formed by the ground plates3 that are the collecting electrodes. There is an inlet `4 for the gas.It is shown at the left in FIG. l. The gas outlet is showndiagrammatically in the form of a pipe 13 at the right of FIG. 2. Themachine has the usual rapping mechanism 5 and collecting bin 6. Currentis supplied to the emitting wires 1 through the frame 7 which carriesthe Wires 1 as usual.

The dust particles in the gas stream entering at the Patented Got. 23,1952 ICC inlet 4 are charged negatively by the ionization of the gasclose to the emitting wires l. The particles tend to gravitate to thegrounded collecting plate 3 under the influence of the electric fieldand give up their charges to the plates. The sum of all these ioniccharges reaching the grounded plates is the total direct current ow inthe precipitator. The higher the voltage on the emitting wire, thegreater the number of negative ions released in the corona and thehigher the current in the precipitator. Since the charge impressed on acharged dust particle is proportionate to the voltage, a high voltage isdesirable for best ei'liciency. As suggested in FIG. 3, the voltage 8 ofthe Wires may be of the order of 50 kv. below the voltage of thegrounded plates.

As already mentioned, the dust particles form a layer of dust 9 on thewalls of the passage, i.e. on the collecting plates 3 (see FIG. 3).

To get from the emitting wires ll to the grounded collecting plates 3,the current must ow not only through the gas in the passage 2 but alsothrough the dust layer 9. In passing through the dust layer 9 thevoltage of course will drop. This voltage drop through the dust layer atany given moment is a function of the resistance of the layer and thequantity of current passing through it. The resistance of the dust layeris a function of its resistivity, and the latter is expressed as theelectrical resistance of a unit cross-section of dust of a unitthickness. The breakdown voltage of a dust layer is customarily given`as volts per centimeter, i.e. that voltage which will arc through a lcnr. thickness of dust. The relation to the current is expressed inusing breakdown voltage as E and resistivity as R. This Will give thecurrent in amperes per square centimeter. Thus expressed, the currentdensity is independent of the thickness of the dust layer.

It the voltage drop through the dust layer becomes Sufciently high, thedust layer will break down and vallow arcs Ito pass from the surface ofthe dust to the plate. Once this happens, the precipitator can becontinued in action only by decreasing the voltage. This Will decreasethe current density in the precipitator and avoid breakdown. Thiscondition can be described as due to high resistivity or to the level ofthe breakdown voltage of the dust layer.

Let us call the resistivity which causes a breakdown of the voltagethrough the dust layer 9 and formation of an arc in a given precipitatorset up, the critical resistivity of that condition.

It has been discovered that the ideal current and voltage is denitelyrelated to the tempe'ature of the gas. This is due to the elect of thegas temperature on the resistivity of the dust layer. In FIG. 4 is shownthe resistivity of ily ash plotted as a curve t2 against the gastemperature. In this typical curve, it will be noted that the curverises up to a maximum `te-mperature between 300" F., T1 and 400 F., T3.The maximum height is at the point `designated T2. If the criticalresistivity is higher than this temperature point T2 then theprecipitator voltage will never have to be lowered because of abreakdown arc in the dust layer. kIn the graph, the temperature passesthe critical resistivity on the Way up to the maximum point T2 fromlower ternperatures at the point designated T1 and curves back downbelow the critical resistivity at a higher temperature point T3. At anytemperature between points T1 and T3 the voltage must be reduced toavoid arc-over.

vIt has been found the Voltage drop in the dust layer is such a majorfactor that by keeping this voltage drop below the breakdown voltage ofthe dust layer, whates ever the gas temperature, no arcs occur betweenthe emitting wires and the collecting plates including the dust layer.

To implement this and to monitor the precipitator and be sure it isValways operating at the maximum possible voltage setting for theparticular conditions, the following continuous Atemperature actuatedvoltage control equipment isV provided. Specically, this may be atemperature sensing unit such as a Moore Nullmatic TemperatureTransmitter. The Moore Nullmatic Temperature'Transmitter consists of agas-filled thermal system acting in conjunction with a vbalancingbellows and an air supply to transmit an air pressure having a directlinear relation to the bulb temperature. The transmitting system usesthe null-balance principle which avoids the effects of bellowsnon-linearity by maintaining the bellows in a very nearly fixed positionfor any value of the measured-temperature. It may be located at theoutlet pipe 13 of a precipitator section or at same other point in thegas stream. The signal from the unit 10 can Abe sent to a modifierstation 14 by compressed air Signal. Here it can be adjusted to lessthan full value if desired. This can bring the sensing unit signal instep with the electrical current situation. j From'thence the modiedsignal is transmitted by air to a Conoflow current controller which hasbeen designated as voltage control equipment 18 in FIG. 5. This isconnected mechanically to the voltage powerstat associated with thetransformer 11. As the gas temperature rises, the voltv`age is loweredand vice versa. The amount of decrease per unit temperature rise is afunction of the setting of the modifier station 14.

nIn initially adjusting the equipment, the modifier station 14 can beset, for example, at 70%. If there is a tendency to arc, the modi-tierstationY should be increased so as to decrease the voltage. -If there isany question whetherY the setting of the voltage has been reduced -toomuch, the modifier station 14 setting can be tried a little lower. Oncethese settings are determined, there should be no necessity for manualchange except under extraordinary conditions, provided the gastemperature range is unchanged. It it isrdesired to return to manualoperation, shut the supply and signal lair valves and i6. If itisdesired to bleed the air system, the bleeder valve 117 can be used.

It will be seen that the dust reaction to temperature change provides anideal way ito anticipate and prevent arcs and to keep the precipitatorat maximum eciency.

What is claimed ist l. In an electric precipitator for cleaning a dustladen gas stream, an inlet -for a dust laden gas stream, an outlet forthe cleaned gas, a passage connected to the inlet through which only thedust laden stream passes, emitting electrodes and grounded collectingelectrodes located in spaced relation in said passage, means to supplydirect current, within the corona range, to the emitting electrodesthereby charging `dust particles in the dust laden gas stream andcausing said particles to form on the collecting electrodes a dustlayerhaving a voltage drop varying according'to the temperature,vincombination with control equipment to vary,V within the corona range,the voltage produced by lthe supply means, and `a gas temperaturesensing unit in the gas stream adapted to activate said controlequipment -to keep the voltage just below the arcover point at differenttemperatures of the dust-layer.

2. In an electric precipitator for cleaning dust laden gas stream, aninlet for a dust laden gas stream, an outlet for the cleaned gas, apassage connected to the inlet through which only the dust laden gasstream passes, emitting electrodes and grounded collecting electrodeslocated in spaced relation in said passage, means to Supply to theemitting electrodes direct current, within the corona range, and havinga voltage just below the arc-over point in the precipitator therebycharging the dust particles in the dust laden gas stream and causingsaid particles to form a dust layer on the collecting electrodes, saidcollecting electrodes having a voltage drop varying according to thetemperature of the dust layer, in combination with control equipmentadapted to lower, within the corona range, the voltage from, or returnsaid voltage to, the voltage of the supply means, and a `gas temperaturesensing unit in the gas stream adapted to activate said controlequipment to keep the lvoltage drop in the dust layer just below thebreak down point of the dust layer at different temperatures thereof;whereby prompt avoidance of arc-over is obtained.

3. In an electric precipitator for cleaning a dust laden gas stream, aninlet for the dust laden gas stream, an outlet for the cleaned gas, apassage connected to the inlet through which only the dust laden streampasses, emitting electrodes and grounded collecting electrodes locatedin spaced relation in said passage, means to supply direct current,within the corona range, to the emitting electrodes thereby charging thedust particles in the dust laden gas stream and causing said particlesto form a dust layer on the collecting electrodes, said dust layerhaving a critical lresistivity below the maximum dust layer arc-overpoint, said :dust layer arc-over point varying above and below thecritical resistivity of the dust layer according to the temperaturethereof, in combination with control equipment adapted to lower thevoltage from, or return said voltage to, the voltage supplied by thesupply means, and a gas temperature sensing unit in the gas streamadapted to activate said control equipment to keep the arc-over point ofthe dust layer below the critical resistivity of said dust layer atdiierent temperatures thereof; whereby prompt avoidance of precipitatorarc-over is obtained.

4. In an electricprecipitator for cleaning a dust laden 4gas stream, aninlet for the dust laden gas stream, an outlet for the cleaned gas, apassage connected to the inlet through ywhich only the dust laden streampasses, emitting electrodesV and grounded collecting electrodes locatedin spaced relation to each other in said passa-ge, means to ysupplydirect current, within the corona range, to the emitting electrodes,thereby charging the dust particles in the dust laden gas vstream andcausing said particles to 'form a dust layer on the collectingelectrodes, said dust layer having a critical break down resistivitybelow the maximum dust layer arc-over point of the direct current, saidarc-over point varying `above and below the critical resistivity of thedust layer according to the temperature thereof, in combination withvoltage control equipment to lower or raiseV the voltage supplied by thesupply means, Vand a temperature sensing unit in the gas stream adaptedto activate the vol-tage control equipment when the gas temperatureindicates the resistivity of the dust layer is labout to lgo above thecritical-resistivity level and to activvate', the Yvoltage controlequipment in the other direction when the `break down point of the dustlayer no longer requires deviation from normal to avoid arcing.

5 .v -An electric precipitator according to claim 4 in which theemitting electrodes are wires and the collecting electrodes are groundedplates.

6. In an electric precipitator according to claim 4 the provisionbetween the temperature sensing unit and the Vvoltage control equipmentof a modifier station adapted to reduce the sensing unit signal to lessthan full value and thereby adjust the magnitude of the voltage change.

References Cited in the le of this patent UNITED STATES PATENTS PenneyFeb. 6, 1940 Berg Apr. 4, 196i

